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Looking at it again, there's the question of accept() behaviour. In the original code, it will call internal_setblocking() on the new fd if the listening socket is non-blocking. In the new code, if SOCK_NONBLOCK is defined it will not call any OS function to set the new fd in non-blocking mode.
Here is what the man page has to say:
 On Linux, the new socket returned by accept() does not inherit file status flags such as
 O_NONBLOCK and O_ASYNC from the listening socket. This behavior differs from the canonical
 BSD sockets implementation. Portable programs should not rely on inheritance or non-inher‐
 itance of file status flags and always explicitly set all required flags on the socket
 returned from accept().
Linux has defined accept4() precisely for this purpose:
 If flags is 0, then accept4() is the same as accept(). The following values can be bitwise
 ORed in flags to obtain different behavior:
 SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the new open file description.
 Using this flag saves extra calls to fcntl(2) to achieve the same result.
 SOCK_CLOEXEC Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See
 the description of the O_CLOEXEC flag in open(2) for reasons why this may
 be useful.